EFFECTS OF TEMPERATURE AND SHEAR HISTORY ON THE NETWORK PROPERTIES OFAMPHIPHILIC HYDROGELS

We report on thermally irreversible hydrogels made from un-cross-linke d hydrophobically modified (hydroxyethyl)cellulose in solution with so dium dodecyl sulfate (SDS). The effects of temperature and shear histo ry on bulk and microstructural properties of these gels were investiga ted over a temperature range 25-70 degrees C. The bulk properties inve stigated were composition, gel volume, and dynamic storage and loss mo duli. Microstructural properties of the networks were investigated usi ng two spectroscopic probes, pyrene and ET(30), which gave complementa ry information about the packing density and surface hydrophobicity, r espectively, of the hydrophobic microdomains in the gels. The hydrogel s exhibit an irreversible transition in both their bulk properties and the structure of the microdomains at a temperature in the range 35-50 degrees C. Below the transition, thermal gelation is observed as the polymer chains expand with temperature and SDS is driven to associate with the side chains. Above the transition temperature the gels collap se, and the dynamic storage modulus of the gels goes down. At the same time the hydrophobic microdomains become increasingly porous. A mecha nism for these changes is proposed, and the properties of these materi als are contrasted with those of other temperature-sensitive hydrogels .